A semiconductor laser device which emits light in a direction perpendicular to the top surface of a base plate can be useful in a two- or three-dimensional integrated circuit. Such a laser device prevents interference between a flat base plate and a divergent output light beam from the laser. A surface-emitting semiconductor laser emitting a light beam from its side surface can be used to emit light perpendicular to its base; however its gain (because of its short resonator) limits its practical use. Another device which can emit light perpendicular to a base plate is described in Appl. Phys. Let. 46 (2), Jan. 15, 1985, pages 115-117, and is illustrated in FIG. 1a of this specification. It comprises an etched-cavity laser emitting a beam in a direction perpendicular to its base plate. As seen in FIG. 1a, a base plate 52 of InP supports an etch-stop layer 54, InP buffer layer 56, GaInAsp activating layer 58, InP cap layer 60 and a mask 62, in the stated order. Selective chemical etching is done in two steps to form an etched cavity having a curved reflective plane 64. After heating the structure at 690.degree.-740.degree. C. the semiconductor laser device depicted in FIG. 1a is obtained. Reference 80 is a heat sink. Such a semiconductor laser device can be made as an integral structure, but it has some disadvantages. For example the manufacturing process is complicated due to the use of two etching steps and it is difficult to form the reflective plane.
When a semiconductor laser is used as a light source, difficulties can arise from mode jumps in the oscillating wavelength due to ambient temperature changes. One prior art technique to stabilize such wavelength changes is depicted in FIG. 1b, where a divergent light beam emitted from the side portion 32 of a semiconductor laser 3 passes through a collimator lens 8 and emerges as a parallel beam. The parallel beam is reflected by an exterior mirror 9 and then passes through the collimator lens 8 again, in the opposite direction, thereby returning to the semiconductor laser. The wavelength can be thereby stabilized, but a large scale optical unit is used, and the stabilization of the oscillating wavelength against mode jumps can be effectively obtained over a wide range of ambient temperature changes.